Translate project content to English

Make English the project-wide language: all comments, string literals,
CLI help text, design docs, journal, agent prompts, and README. Only
CLAUDE.md (the user's own instruction file) stays German, and the live
conversation between user and Claude continues in German.

Adds a new "Project language: English" section to docs/DESIGN.md as
the durable convention. No logic changes — translation only. Four
internal error strings in the typechecker were retranslated; no
test asserts on their wording.

Verified: cargo test --workspace passes (44/44).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ailang-core/src/ast.rs

@@ -1,4 +1,4 @@
-//! AST-Knoten. Serde-Layout entspricht dem JSON-Schema in `docs/DESIGN.md`.
+//! AST nodes. Serde layout matches the JSON schema in `docs/DESIGN.md`.
 
 use serde::{Deserialize, Serialize};
 
@@ -36,14 +36,14 @@ impl Def {
     }
 }
 
-/// Externer Helper: Name einer Definition (für Tools wie `ail diff`,
-/// die den Def-Knoten entkoppelt vom Methoden-Aufruf konsumieren).
+/// External helper: name of a definition (for tools like `ail diff`
+/// that consume the def node decoupled from the method call).
 pub fn def_name(def: &Def) -> &str {
     def.name()
 }
 
-/// Externer Helper: Diskriminator-Tag einer Definition (`fn`, `const`, `type`).
-/// Identisch mit dem `kind`-Feld in der JSON-Repräsentation.
+/// External helper: discriminator tag of a definition (`fn`, `const`, `type`).
+/// Identical to the `kind` field in the JSON representation.
 pub fn def_kind(def: &Def) -> &'static str {
     match def {
         Def::Fn(_) => "fn",
@@ -113,8 +113,8 @@ pub enum Term {
         op: String,
         args: Vec<Term>,
     },
-    /// Konstruktor-Anwendung. `type_name` bindet die ADT an, `ctor` den Variant.
-    /// Beispiel: `Some(42)` -> `{ "t": "ctor", "type": "Option", "ctor": "Some",
+    /// Constructor application. `type_name` binds the ADT, `ctor` the variant.
+    /// Example: `Some(42)` -> `{ "t": "ctor", "type": "Option", "ctor": "Some",
     /// "args": [{"t":"lit","lit":{"kind":"int","value":42}}] }`.
     Ctor {
         #[serde(rename = "type")]
@@ -123,7 +123,7 @@ pub enum Term {
         #[serde(default)]
         args: Vec<Term>,
     },
-    /// Pattern matching über einen Wert.
+    /// Pattern matching over a value.
     Match {
         scrutinee: Box<Term>,
         arms: Vec<Arm>,
@@ -139,13 +139,13 @@ pub struct Arm {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(tag = "p", rename_all = "lowercase")]
 pub enum Pattern {
-    /// `_` — bindet nichts, matcht alles.
+    /// `_` — binds nothing, matches everything.
     Wild,
-    /// `x` — bindet den Wert an einen Namen.
+    /// `x` — binds the value to a name.
     Var { name: String },
-    /// Match auf ein Literal.
+    /// Match on a literal.
     Lit { lit: Literal },
-    /// Match auf einen Konstruktor mit Sub-Patterns für seine Felder.
+    /// Match on a constructor with sub-patterns for its fields.
     Ctor {
         ctor: String,
         #[serde(default)]